This invention relates to a luminous display device comprising means for modifying at will the reflectivity of at least a portion of a surface so as to vary the luminance and/or the color of that part of the surface whose reflectivity has been changed. The device permits the display of patterns corresponding to those parts of the surface in which the reflectivity undergoes variation and makes it possible in more general terms to modify the coefficient of reflection of a surface by creating or removing a reflecting layer which forms one face of an interferential filter and consists of a transparent film having a given optical thickness and tightly held between two partially reflecting faces.
Numeric and alphanumeric display devices can be placed in two categories: light-emitting devices (electroluminescent diode, gaseous discharge device, incandescence device) and the devices which call for auxiliary lighting such as liquid crystals, the electrochromic and electrolytic devices. The optically passive visual display devices which are invisible in darkness and fall in the category just mentioned consume much less energy and their visibility is enhanced by a high level of illumination, which can be an advantage in the case of observation in an environment of high luminous intensity. A further property of electrochromic and electrolytic devices lies in the fact that they consume energy only at the time of modification of the information which is displayed. Such devices essentially have an internal memory and their use is of interest in all applications in which the switching frequency is of low value and the available power is limited.
In conventional display devices which fall in the category of electrolytic devices, action is produced on the variation in reflectivity of a glass plate when a colored metallic or organometallic film-layer is deposited by electrolysis on the face remote from the viewing side. It is clearly possible to work both in reflection and in transmission. However, the contrast obtained with a metallic film-layer is sufficient only in the case of thicknesses of a few hundred Angstrom and therefore entails a large amount of electric power in order to deposit the metallic film-layer on the pattern design which is to be represented. Electrolytic display devices of the prior art do not employ the constructive interference phenomenon which is characteristic of interferential filters.